Hi,
As Otavio pointed out, d-i is meant to install system to fresh formatted
partition. This does not mean that you can not use d-i to migrate your
system to new Debian with some skill.
Since you are an experienced admin, I suspect you were doing this fresh
install to an empty partition while copying many configuration files and
data from old system partition. Are you sure you did this configuration
data migration properly? It does take careful attention to the detail.
As I vaguely remember, Some Ubuntu installations do no set up root
password but lets user login via sudo as default. Debian does not set
up sudo as installed. So if you copied /etc/passwd and /etc/shadow from
old Ubuntu to new Debian system, you are likely loose password login to
root since Ubuntu file is missing it and you do not have sudo and its
required setup.
FYI: you must be careful about variable system group ID between 101-999
when migrating system with manual copying. I just got hit by this from
lenny to squeeze upgrade.
Osamu
(You may be able to upgrade Ubuntu to Debian without d-i but toth
dist-upgrade by pointing URL in /etc/apt-sources.list to newer Debian
version. This Ubuntu->Debian conversion method is not guaranteed nor
supported to work.)